The present invention relates to a heat-sensitive recording material which makes a record of various types of information by thermal means, and more particularly to improvements in the chromogenic function of said recording material.
In certain types of information equipment, such as facsimilies, printers, recorders or the like, a system in which a record is made of information by thermal means has been developed in recent years. Keeping in step with the advent of such a system, it has been proposed to make use of a variety of heat-sensitive recording materials. Among others, currently noteworthy is a heat-sensitive recording material comprising a carrier or substrate having thereon a heat-sensitive layer composed mainly of a leuco dye that is colorless in its normal state, a developer such as a phenolic compound and a water-soluble binding agent.
The article "Berischte der deutschen chemischen Gesellschaft" by O. Fisher, F. Romer et al, 42.2934 (1909) describes a reaction between a chromogenic leuco dye, eg., crystal violet lactone and a phenolic compound resulting in color development. Color development caused by thermal means is also known from, for example, U.S. Pat. No. 3,539,375 and Japanese Patent Publication No. 14039/1970 specifications.
Usually, the heat-sensitive recording material of the type as mentioned just above is prepared by dispersing individually the leuco dye and the developer serving as a color-forming component in separate aqueous media containing a water-soluble resin acting as a binding agent, mixing together the resulting dispersions, and applying the mixture on a carrier followed by drying. A variety of dispersing means, such as ball mills, attritors, sand grinders or the like for fine pulverization, may be used. In principle, the recording material is then subjected to color development by softening or fusing the leuco dye, phenolic compound and water-soluble binding agent with the application of heat to bring both color-forming components in contact for a chromogenic reaction. In the actual embodiment, the dispersed particles preferably have a particle size as small as possible, e.g., on the order of several microns. Otherwise the heat-sensitive layer may become coarse-grained or rugged, thus decreasing the chromogenicity and the resolving power in addition to causing a lowering of quality. It is also important to provide isolated dispersions of particles without producing any foam during dispersion. Insufficient isolation and dispersion of particles present similar problems in respect of the chromogenic function and the resolving power, and may be responsible for the phenomenon referred to generally as primary color development. As will be understood from the foregoing, the quality of the heat-sensitive recording material is dependent largely upon the dispersibility of the color-forming components used.
Since the processing speed of various types of information equipment has increased, on the other hand, there is now a strong demand for heat-sensitive recording materials having a higher level of sensitivity. To this end, various attempts have been made to improve the sensitivity to heat of the recording material by the addition of diverse substances into the heat-sensitive layer in order to supplement the leuco dye, phenolic compound and binding agent. These substances include, for example, urea, phthalic anhydride, acetanilide (Japanese Patent Publication No. 4160/1698), waxes (Japanese Laid-Open Publication No. 19231/1973), nitrogen-containing compounds (Japanese Laid-Open Publication No. 34842/1974), acetoacetic anilide compounds (Japanese Laid-Open Publication No. 106746/1977), coumarone compounds (Japanese Laid-Open Publication No. 26139/1978) and the like. Although the mechanism of the sensitizing effect exerted by these substances has not yet been revealed, it can be assumed that they are first thermally fused at low temperatures causing the leuco dye or the phenolic compound to be molten and enhancing thereby the reaction between the color-forming components coming in contact with each other. Alternatively the mixture of the leuco dye and phenolic compound with the additional substances may have a eutectic point lower than the melting point of any one of the components, causing the color-forming components to be brought into contact at a faster rate and leading to increased heat-sensitivity. In any event, these substances are essentially differentiated from the color-forming ingredients, and are thermally fusible materials that take no direct part in the chromogenic reaction, and have a relatively low melting point.
In the use of these substances, however, difficulties are still encountered. In other words, some of the known substances produce a poor sensitizing effect while other produce a good sensitizing effect, but show an undersirable shelf life over an extended period of time thanks to the facile occurrence of liquid coloration or primary color development (this is especially true of water-soluble substances such as urea). In addition, certain types of these substances are so poor in dispersibility that they are not readily dispersed to fine particles or easily foamed. The conventional substances also pose an economic problem. Thus, no satisfactory substances are known.